FIG. 1 illustrates a prior art photomask having a shadow pattern as described in Korean Patent Publication No. 2002-0090487, which is hereby incorporated by reference in its entirety. The photomask 200 of FIG. 1 has a shadow pattern 210. The photomask 200 comprises a low density pattern region 202 having a low ratio of pattern areas 201 per unit area, a high density pattern region 203 having a high ratio of pattern areas 201 per unit area, and an interface 205 between the low density pattern region 202 and the high density pattern region 203. The shadow pattern 210 is positioned in the interface 205 and includes a slit having a width (w) which will not pass light from a predetermined light source.
If the light source is an excimer laser, the shadow pattern 210 preferably has a width (w) less than 40 nm so that it will not pass light from the excimer laser source. The width (w) is determined by the type of the light source. For example, the shorter the wavelength of the light source, the narrower the width. The shadow pattern 210 functions as a barrier that prevents light from a light source to which the low density pattern region 202 is exposed from interfering with the high density pattern region 203. Therefore, the problem that the high density pattern region 203 is changed in terms of its shape or formed in an unintended region by interference from the light source for the low density pattern region 202 may be solved. Furthermore, since the shadow pattern 210 can be simultaneously formed with the pattern 201 during a photomask fabricating process, conventional equipment and methods for exposure may be advantageously utilized to form the shadow pattern 210 without any additional effort. As a result, the method as described above is cost-effective.
Widely used prior art masks include a binary mask comprising quartz and Cr, a half tone phase shift mask comprising quartz, Cr and MoSiN, and a Cr-less mask capable of improving a mask resolution by changing the phase of the quartz. The half tone phase shift mask is more advantageous than the binary mask in terms of resolution and process margin. Consequently, the half tone phase shift mask has been widely used in the critical lithography process for obtaining a 0.13 μm linewidth or a 0.09 μm linewidth. KrF (248 nm exposure wavelength) lithography equipment can form a pattern having a gate line with a 0.13 μm linewidth or a contact hole with a 0.16 μm linewidth. However, for a pattern having a linewidth less than 0.13 μm, KrF lithography has difficulties in ensuring a sufficient process margin or even forming the pattern itself. Thus, ArF (193 nm exposure wavelength) lithography or alternative methods are required to solve the problem. On the other hand, the Cr-less mask can improve the mask resolution, but cannot be formed as a pattern which has an entire region free of Cr. Accordingly, it is unsuitable for commercial use. Moreover, known Cr-less masks are also disadvantageous in that unintended patterns are formed in the transition region where the phase of the light source shifts from 0 degrees to 180 degrees.